When designing electronic equipment, regulatory agencies have set several specifications or standards which should be met. The electrical outlet provides an ac voltage that has a waveform conforming to standards of magnitude, frequency and harmonic content to electrical equipment. However the current drawn from the outlet is determined by the characteristics of the electrical equipment which receives the ac voltage. Regulatory agencies set standards for particular characteristics of the current that may be drawn from the ac electrical outlet. For example, a standard may set limits on the magnitudes of specific frequency components of the ac current. In another example, a standard may limit the rms value of the current in accordance with the amount of power which the outlet provides. One standard places limits on the power factor correction (PFC) which should be included for electronic devices, such as for example the International Electrotechnical Commission (IEC) standard IEC 61000-2-2. Power factor is particularly important for power distribution systems. When electronic equipment (such as a power supply) has less than unity power factor, power utilities would need to provide the electrical equipment with more current than electrical equipment with unity power factor. By employing PFC, power utilities may avoid the need for extra capacity to deliver current.
The power factor is the ratio of the average power over a cycle to the product of the root mean square (rms) voltage and the rms current. The power factor has a value between zero and one with unity power factor as the ideal case. Generally, a PFC circuit shapes the input current waveform as closely to the input voltage waveform in an attempt to achieve unity power factor.
One example of electrical equipment which may utilize a PFC circuit is a switched-mode power supply. In a typical switched mode power supply, the power supply receives an input from an ordinary electrical outlet. Switches in the power supply are switched on and off by a control circuit to provide a regulated output. Since the power supply which receives the ac voltage determines the characteristics of the ac current, power supplies often use active circuits at their inputs to maintain a high power factor. Conventional power factor corrected power supplies may be designed in two stages. The first stage is the PFC circuit which attempts to shape the input current waveform to achieve unity power factor. The second stage is the switched-mode power supply which provides a regulated output.
In general, a step-up converter may be utilized as a PFC circuit. In particular, a boost power converter may be utilized as a PFC circuit. However, boost converters typically have a fixed output voltage regardless of the value of the voltage delivered by the power utilities, or in other words regardless of the line input voltage. Generally, different countries have different standards for the amount of ac voltage which is delivered. The ac line voltage may vary from 85 to 265 V ac and typical step-up converters utilized for PFC may have an output between 380-400 V dc. However, for countries with lower ac line voltages it may be desirable for the PFC circuit to provide an output voltage less than 380-400 V dc.